New Imaging Data-reduction Recipes for ORAC
Malcolm Currie
Joint Astronomy Centre, Hilo, Hawaii
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(POL_JITTER).
The former will usually yield more accurate results, but is less efficient
at the telescope. These recipes flat field using an existing flat,
then extract the o- and e-beam regions for the target and the sky.
The sky-subtracted target regions are registered and form a mosaic
for each waveplate angle and beam. The recipe registers the eight
mosaics, before deriving the polarisation parameters at each pixel.
The polarisation information is 3 x 3 binned (without undersampling
under normal seeing), and noisy data excluded to form a catalogue
of vectors in FITS format. If you select the KAPPA (GWM) display to
present say the intensity mosaic, the vectors appear overlaid. The
picture of M87 below shows an example of how this appears. The observer
can adjust the binning size and vector signal-to-noise rejection criteria
through arguments to the _CALC_STOKES_ script called by the above
recipes. Another option is to use POLPACK commands offline to examine
the reduced data. For extended (>35" with UFTI, >8"
with IRCAM) sources, chopped blank sky frames are necessary. |
In essence the recipe above calculates
[(#1-#2)-(#4-#3)] + [(#5-#6)-(#8-#7)]
divided by the flat field after dark subtraction of each frame
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he number and variety of ORAC-DR recipes for imaging continues
to grow. The ORAC-DR pipeline permits concurrent assessment of the
images obtained while collecting data. This is especially important
for specialised techniques such as polarimetry and Fabry-Perot imaging.
Polarimetry with IRCAM and UFTI
There are now five polarimetry recipes in three groups: flat-field
creation, point and extended sources. The point-source recipes permit
a choice of whether to measure all four waveplate angles before
moving to the next spatial jitter position on the sky (recipe POL_ANGLE_JITTER),
or to jitter to all positions before rotating the waveplate
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